KANSAS CITY, Kan. A few years ago, the League of Women Voters held a three-week voter registration drive at Washburn University in Topeka, Kansas. Some 400 people stopped by during the drive, but only about 75 followed through and became registered voters.

The low follow-through rate fit into an alarming pattern observed by the League of Women Voters ever since Kansas began requiring people to prove their citizenship when they registered to vote. After that state law was implemented in 2013, registration efforts at high schools and on college campuses became remarkably less effective, according to the league. Students, after all, tend to lack easy access to proof-of-citizenship documents like a birth certificate or a passport.

Margaret Ahrens, the former president of the Kansas League of Women Voters, told a federal court on Wednesday that it was like nothing her group had ever seen before. She called the law a dead hit on voter registration drives.